1. Field of the Invention
The present invention relates to a lithium battery and a method of producing a positive-electrode active material for lithium batteries.
2. Description of the Related Art
With the advance of portable electronic appliances, the development of high-performance batteries has been required in recent years. A lithium ion battery, in which a carbon material is used in its negative electrode, and lithium cobaltate is used as stratiform composite oxide in its positive electrode, has been put into practice as a non-aqueous solution battery which has high operating voltage and high energy density. However, since lithium cobaltate is rare in terms of resources and expensive, lithium- manganese composite oxide or lithium nickelate has been proposed as a substitute for lithium cobaltate. Lithium- manganese composite oxide has a problem that not only is theoretical capacitive density low but also reduction in capacity becomes remarkable as the charging/discharging cycle is repeated.
On the other hand, lithium nickelate (lithium-nickel oxide) is a stratiform compound having the same crystalline structure as lithium cobaltate which is put into practice. In the lithium nickelate, lithium is inserted between layers of NiO6 octahedrons having edges in common. A method of producing lithium nickelate generally comprises the steps of: mixing Ni(NO3)2, Ni(OH)2, NiCO3, NiO, NiOOH, etc. used as a nickel source and LiOH, LiNO3, Li2CO3, Li2O2, etc. used as a lithium source; and heating the mixture at a temperature range of about 600xc2x0 C. to about 900xc2x0 C. in an oxygen stream.
The structure of lithium nickelate is, however, similar to a rock salt structure as reported in Solid State Ionics, 44, 87, 1990, Chem. Express, 7, 689, 1992, or the 33rd Battery Discussion Lecture Summary, p.21, 1992. Accordingly, there is a problem that the capacity is reduced because nickel and lithium are easily replaced by each other so that a partially disordered structure is generated.
Further, there is an attempt to use nickel oxyhydroxide as a nickel raw material. The attempt is disclosed in Japanese Patent Unexamined Publication No. Sho-63-19760. In the Japanese Patent Unexamined Publication No. Sho-63-9760, there is a proposal in which nickel oxyhydroxide containing cobalt in a range of from 20% to 75% is used as a lithium battery active material. In order to attain improvement in discharging characteristic, Japanese Patent Unexamined Publication No. Hei-6-310145 has proposed a method in which hydroxide or oxide containing trivalent nickel ions is mixed with a lithium salt and then, the mixture is heated. According to this Japanese Patent Unexamined Publication No. Hei-6-310145, a sodium hypochlorite aqueous solution, an aqueous solution of chlorine or an aqueous solution of bromine is made to react with a sodium hydroxide solution containing dispersed divalent nickel hydroxide (Ni(OH)2) to thereby produce nickel oxyhydroxide. After the hydroxide or oxide containing nickel oxyhydroxide is mixed with lithium nitrate, the mixture is pressed, molded, dried and heated in air at a temperature range of 600xc2x0 C. to 800xc2x0 C. Then, the mixture is pulverized, molded and heated again in air at a temperature range of 700xc2x0 C. to 900xc2x0 C. so as to be sintered to thereby produce lithium nickelate.
It is, however, difficult to produce pure lithium nickelate by these methods. Particularly, there is a large defect that not only the voltage in charging/discharging characteristic varies multistageously, for example, in four stages but also the high-rate discharging performance is lowered. Japanese Patent Application No. Hei-7-129663 has proposed a synthesis method to solve this problem. In this method, cobalt-containing nickel oxyhydroxide is heated while lithium nitrate is made to act on the cobalt-containing nickel oxyhydroxide to thereby synthesize lithium nickelate exhibiting a uniform charging/discharging reaction.
As one attempt of a low-temperature synthesis method, which is not such a high-temperature synthesis method by solid-phase reaction, there is a synthesis method using ion exchange as proposed in Japanese Patent Unexamined Publication No. Hei-6-349494. There is, however, no demonstration concerning lithium nickelate. Furthermore, it is difficult to synthesize pure xcex2-NiOOH which can be made to be a starting material for synthesis of lithium nickelate and, consequently, it is difficult to obtain lithium nickelate of high purity.
On the other hand, production of lithium nickelate not by a chemical synthesis method but by an electrochemical method is described in Soviet Electrochem., 6, 1268, 1970; GS News, 37, 84, 1978; and GS News, 45, 23, 1986. In those publications, however, only the behavior as positive electrodes for an alkali battery is disclosed.
Japanese Patent Unexamined Publication No. Sho-63-19761 shows an example of applying an electrochemically produced lithium nickelate to a lithium battery. In the Japanese Patent Unexamined Publication No. Sho-63-19761, use of nickel hydroxide charged in a lithium hydroxide solution as an active material is proposed. In this method, however, it is necessary to control the producing process strictly in order to obtain a stable active material.
As described above, a method comprising the steps of: mixing a nickel compound with a lithium compound; and heating the mixture at a temperature of 600xc2x0 C. to 900xc2x0 C. in an oxygen atmosphere is a general method for synthesizing lithium nickelate. However, partially disordered structure produced at a high temperature becomes a serious problem. Furthermore, there is a large defect that not only the voltage in charging/discharging characteristic varies multistageously, for example, in four stages but also the high-rate discharging performance is lowered. Accordingly, lithium nickelate does not serve as a substitute for lithium cobaltate having the same stratiform structure. From the point of view of electrode reaction, this is considered to be caused by the fact that the diffusion of lithium ions with the charging/discharging reaction is difficult and is not performed evenly in lithium nickelate.
Further, an attempt to synthesize lithium nickelate at a low temperature has been made in order to obtain lithium nickelate having a uniform structure. It is, however, difficult to obtain a uniform sample because reactivity is lowered when the temperature is low in a solid-phase burning method. Accordingly, the proposal of a low-temperature synthesis method using a simplified process different from the high-temperature synthesis method by solid-phase reaction has been required and, from the point of view of safety and cost, the low-temperature synthesis method is considered to be preferred. However, no useful synthesis method has ever been established.
It is an object of the present invention to provide a synthesis method of lithium nickelate where the raw material is easily obtainable and where the production is simplified by using a low-temperature synthesis method, and further to provide an active material having a stable charging/discharging performance and a lithium battery using such an active material.
The positive electrode active material for lithium batteries according to the present invention comprises the step of chemically oxidizing a compound represented by the chemical formula HxLiyMO2 in a solution containing lithium ions, where 0xe2x89xa6xxe2x89xa62, 0xe2x89xa6yxe2x89xa62, 1 less than (x+y)xe2x89xa62, and M is one or two kinds of transition metals selected from Co and Ni. Preferably, the compound is nickel hydroxide; the compound is nickel hydroxide containing nickel oxyhydroxide; the cobalt content of the compound is in a range of from 2 to 90 mol % {Co/(Ni+Co)}; the solution containing lithium ions has lithium hydroxide and water as constituent elements; and/or peroxodisulfate is used as an oxidizing agent in the oxidizing step.
As described above, because the positive electrode plate using the lithium battery active material according to the present invention can be formed not from nickel xcex2-oxyhydroxide which is difficult to synthesize but from available nickel hydroxide as a starting material and no high-temperature treating process is required, an inexpensive and simple producing method can be provided. In addition, the obtained active material shows a good cycling performance and a monotonous variation of the potential with charge and discharge. Accordingly, the industrial value of the present invention is very large.